1. Field of the Invention
The present invention relates to a diagnostic device for a catalytic converter which is employed for treating exhaust gas emitted from an internal combustion engine. More particularly, it relates to a catalyst-deterioration diagnostic system which can diagnose the state of a catalyst by the use of the preset operating-state index of an internal combustion engine and can report the diagnosed result through a display, an alarm sound or the like to the driver of a vehicle having the system installed therein.
2. Description of the Related Art
It has been known that, in making the exhaust gas of an internal combustion engine innoxious through oxidizing and deoxidizing reactions, effectively the reactions are promoted by employing a catalyst which contains a plurality of kinds of catalytic components. In this regard, when the catalyst is used for a long term, it deteriorates into a lower conversion efficiency. Therefore, such a step has heretofore been taken that the conversion efficiency of the catalyst is checked mainly at the time of a routine inspection, and that a catalytic converter including the catalyst is replaced when the regulated value of the conversion efficiency is not met. Accordingly, the conversion efficiency is not especially checked till the routine inspection, and a vehicle in which the catalytic converter is installed runs under the unsatisfactory innoxious condition of the exhaust gas in some cases.
Such circumstances unfavorably lead to environmental pollution. Attempts have therefore been made to check the performance or activity of the catalyst at all times by any method, and to actuate a warning device or a countermeasure device when the catalyst is abnormal. Examples will be stated below.
Japanese Patent Application Publication No. 2896/1982 discloses a technique wherein oxygen-percentage detectors are respectively mounted before and behind a catalytic converter so as to diagnose the state of the catalytic converter on the basis of the detected values of both the detectors. Concretely, the detectors capable of detecting the oxygen concentrations of exhaust gas are respectively disposed upstream and downstream of the catalytic converter. When the oxygen concentration of the exhaust gas detected by either of the detectors has abnormally fallen outside a preset range until the difference thereof from the output of the other detector has exceeded a preset magnitude, a signal is generated to actuate an alarm device or a device for taking steps to rectify the abnormality.
In addition, according to a technique entitled “Apparatus for Detecting Deterioration of Catalyst” as disclosed in Japanese Patent Application Laid-open No. 249320/1991, the correlation coefficient of the output signals of oxygen sensors mounted before and behind a catalytic converter is calculated so as to diagnose the state of a catalyst with the calculated correlation coefficient.
Further, a method which adopts a sensor for diagnosing the deterioration of a catalyst is disclosed in Japanese Patent Application Laid-open No. 165744/1988. With this method, the catalyst for combustion is so constructed that a catalytic component is carried on a support having a large number of through holes. A basic material in the shape of a rod is inserted into the through hole, and is used as the sensor. The state of the catalyst is diagnosed by measuring the resistance of the rod across both the ends thereof. This method can diagnose the state of the catalyst irrespective of the operating conditions of an engine.
In general, however, the activity of a catalyst varies greatly in dependency on the temperature thereof as illustrated in FIG. 12 of the accompanying drawings, so that the conversion efficiency etc. of the catalyst varies in accordance with the temperature thereof. It is accordingly necessary for the precise diagnosis of the deterioration state of the catalyst to stipulate the temperature of the catalyst on the occasion of the diagnosis or to correct a calculated conversion efficiency in accordance with the temperature of the catalyst at the time of the diagnosis. To this end, however, a temperature sensor for monitoring the temperature of the catalyst is required, resulting in the problem of rise in cost.